The cushioning function of absorbing and alleviating the shock at landing is demanded in shoe soles, in addition to the lightness in weight and the function of supporting the foot stably. Recently, shoe soles having the repulsion function (rebound function) in addition to the above-mentioned functions have been presented. The repulsion function refers to the function of storing the impact energy at landing as deformation energy and emitting the energy of deformation when disengaging from the ground. This function is useful for improving exercise ability of a wearer.
By compressing or bending an element of the shoe sole, the deformation energy is stored in the element. However, when viscoelastic material having a small Young's modulus such as foamed resin used for a cushioning member of the shoe sole is deformed, energy is dissipated as heat and so on. Accordingly, generally, such viscoelastic material cannot perform the repulsion function sufficiently.
The configurations of shoes having the above-mentioned repulsion function are disclosed in the following patent documents.
First patent document: Japanese Utility Model Registration No. 3082722
Second patent document: Japanese Utility Model Registration No. 3053446
Third patent document: Japanese Patent Laid Open No. 02-114905
Fourth patent document: Japanese Patent Laid Open No. 01-274705
Fifth patent document: Japanese Patent Laid Open No. 2004-065978
Sixth patent document: Japanese Utility Model Registration No. 3093214
Seventh patent document: WO96/38062 (Japanese National Phase PCT Laid Open Publication of No. 11-506027)
The first and second patent documents disclose shoes with an improved repulsion function. In the first and second patent documents, the repulsion function is improved by attaching a repulsive member, which is obtained by forming an elastic material in the shape of a tube, to a bottom surface of the shoe sole. However, since such repulsive members have substantially the same size as the foot and supports the whole of the foot with a curved surface, it cannot support the foot stably.
FIG. 14(a) is a side view of a shoe disclosed in the third patent document. As shown in this figure, a spring 101 of generally oval cross-section is attached to a midsole 100 at a heel part of the shoe.
However, this spring 101 is accommodated in the soft midsole 100. Accordingly, most part of impact energy (shock energy) at landing is absorbed and dissipated in the midsole 100, and the remainder of the energy is absorbed by the spring 101. Accordingly, the amount of energy stored by the spring 101 is reduced.
In addition, impact load (shock force) of landing is applied to the oval spring 101 after having been dispersed in the midsole 100. Accordingly, since the dispersed impact load is applied on each part of the oval spring 101 as distributed load, the amount of deflection of the endless spring 101 is considered to be small. Therefore, impact energy cannot be stored in the oval spring 101 sufficiently.
FIG. 14(b) is a side view showing a partially notched shoe disclosed in the fourth patent document. As shown in this figure, a cavity 103 is formed in the shoe sole. A reaction plate 104 is built in this cavity 103. The reaction plate 104 has upper and lower facing sides 104a, 104a and fore and rear curved parts 104b, 104b that connect the upper and lower facing sides 104a, 104a. A gel cushioning member 105 is provided in the reaction plate 104.
Since the reaction plate 104 is accommodated in the shoe sole also in the shoe disclosed in this patent document, the shoe has similar demerits to the shoe of the third patent document. It is supposed that the part, in which deformation energy due to shock at landing is stored, is mainly the fore and rear curved parts 104b, 104b, not the upper and lower facing sides 104a, 104a. 
FIG. 15(a) is a side view showing configuration of a shoe sole disclosed in the fifth patent document, and FIGS. 15(b) and 15(c) are enlarged perspective views of a deforming member thereof.
The shoe sole of the fifth patent document has a plurality of honeycomb deforming members 106. When the shoe sole is compressed vertically, the deforming members 106 deform from the state shown in FIG. 15(b) to the state shown in FIG. 15(c). At this time, a tension member 107 of the deforming member 106 is stretched, thereby to store energy therein. However, the energy stored in the member due to stretching is much smaller than the energy stored in the member due to bending. Therefore, this shoe sole also cannot store energy sufficiently.
FIG. 16(a) is a side view of a shoe disclosed in the sixth patent document.
In this figure, a depressed part 121 is formed at a part of the midsole 120 corresponding to the heel, and a cushioning member 121 made of plastic is disposed to the depressed part 121. The cushioning member 122 is formed to be tubular in the shape of a letter “D” from the side view. A circular-arc arch part 123 and a flat bottom plate part 124 integrally constitute the cushion member 122. A venting cavity 125 is formed between the arch part 123 and the flat plate part 124.
In this shoe, the bottom plate part 124 of the cushion member 122 is flat-shaped. Accordingly, even if the shock at landing is applied to the shoe sole from below, the bottom plate part 124 does not perform bending deformation.
FIG. 16(b) is a sectional view of a shoe sole disclosed in the seventh patent document.
As shown in the figure, a cavity 131 is formed in an insole body 130. A plate 132 and an insert 133 are accommodated in the cavity 131. The insert 133 has a V-shaped part consisting of a heel lever 134, a fulcrum 135 and a base 136. During heel strike, localized shock force is applied to a heel region 137, thereby to enhance the energy return characteristics of the insert 133.
In this prior art, since the heel region 137 corresponding to the V-shaped part of the insert 133 protrudes downwards, shock force is easy to be absorbed by the insert 133.
However, since the insert 133 is V-shaped, when a load F1 is applied obliquely from below the shoe at an initial landing of the foot, the base 136 is easy to be compressed in the longitudinal direction of the plate and to buckle. Accordingly, in the case where the load F1 is applied obliquely from below the shoe, the base 136 is hard to perform bending deformation. Further, bending deformation does not occur in a part of the heel lever 134 forward of the fulcrum 135. That is, the part of the heel lever 134 cannot absorb shock or store energy.
Moreover, with the configuration shown in this figure, in the foot-flat stance where the whole of the foot touches the ground, the insert 133 bends, thereby to return stored energy. However, in the period during which the initial landing is shifted to the foot-flat stance, energy cannot be stored sufficiently and therefore cannot be returned sufficiently.